In vehicles using electric traction motors, alternating current (AC) motor drives are used to provide a requested torque to the motor shaft. Most motor drives attempt to provide a balanced set of purely sinusoidal currents to the motor stator windings to produce a constant torque with no distortion or ripple. However, due to practical design constraints of the AC motor, torque ripple exists even with purely sinusoidal stator current excitation. Torque ripple may cause speed ripple, excite driveline resonances, or produce other undesirable effects. In the case of a vehicle, torque ripple may produce vehicle oscillations or noise.
In some situations, the torque ripple generated by a motor can be reduced by making mechanical changes to the motor design, such as the winding configuration, stator tooth geometry, rotor barrier geometry, and rotor skewing. However, there is a trade-off between torque ripple and torque density of the motor. Therefore, in all practical applications, the motor produces some torque ripple when supplied by a sinusoidal current. Passive damping methods, such as the addition of structural reinforcement or sound dampening materials to the vehicle, may be utilized to reduce some of the adverse affects of torque ripple and mitigate acoustic noise. However, these damping methods can be costly and do not directly address the problem of torque ripple produced by the motor.
In the case of hybrid or electric vehicles, higher level supervisory controllers may employ algorithms such as active damping which attempt to modulate the torque command provided to the AC motor drive in order to minimize excitation of driveline resonances due in part to the torque ripple. However, these algorithms typically cannot operate at very low speeds, as they can not differentiate between the modulation of the driver requested torque command and oscillations induced by the torque ripple of the AC motor. Alternative techniques attempt inject a harmonic cancellation current in the fundamental synchronous frame. However, these techniques fail to address the effects of current regulator bandwidth limitations on the system, which could possibly lead to increased torque ripple.